The present invention relates to a gear train, and relates particularly to the configuration of a gear train having a planetary gear train mechanism suitable for configuring a small speed reducer.
Speed reducers using a planetary gear train mechanism are widely used in the drive mechanism of various types of mechanical devices because they are generally compact and achieve a high gear reduction ratio. An example of this type of speed reducing apparatus is the compact speed reducer disclosed in Japanese Patent Laid-Open Publication (kokai) H2-31047. This compact speed reducer has a sun gear, a planetary gear meshing with the sun gear, a fixed internal tooth gear meshing with a first gear part of the planetary gear, and a movable internal tooth gear coaxial to the fixed internal tooth gear and meshing with a second gear part of the planetary gear. The first gear part meshing with the fixed internal tooth gear and the second gear part meshing with the movable gear part are disposed coaxially and mutually adjacent in the axial direction, and are described in kokai H2-31047 as having the same number of teeth. The first gear part and second gear part of the planetary gear, however, generally have different numbers of teeth.
The gear reduction ratio can be greatly increased in a gear train having this type of planetary gear train mechanism without incurring an increase in the size of the mechanism. For example, if in the above-cited mechanism e is the tooth count of the sun gear, z1 is the tooth count of the first gear part of the planetary gear, z2 is the tooth count of the second gear part of the planetary gear, I1 is the tooth count of the fixed internal tooth gear, I2 is the tooth count of the movable internal tooth gear, the sun gear is the input part, and the movable internal tooth gear is the output part, the gear reduction ratio can be written as
r={1+(I1/e)}/{1xe2x88x92(z2/z1)*(I1/I2)}xe2x80x83xe2x80x83(1) 
As described in kokai H2-31047, if z1=z2, sun gear tooth count e=6, fixed internal tooth gear tooth count I1=60, and movable gear tooth count I2=61, the difference in the tooth counts of the fixed internal tooth gear and movable internal tooth gear is 1, and the gear reduction ratio is 671. If the other tooth counts remain the same and the tooth count I2 of the movable internal tooth gear is 62, the tooth count difference is 2 and the gear reduction ratio is 341. If the other tooth counts remain the same and the tooth count I2 of the movable internal tooth gear is 63, the tooth count difference is 3 and the gear reduction ratio is 231, and if the tooth count I2 of the movable internal tooth gear is 64, the tooth count difference is 4 and the gear reduction ratio is 176.
However, if the difference between the tooth count of the fixed internal tooth gear and the tooth count of the movable internal tooth gear is 1 in a gear train using a planetary gear train mechanism in which the tooth counts z1 of the first gear part and z2 of the second gear part of the planetary gear are the same as described above, the number of planetary gears that can be assembled between the sun gear and the fixed internal tooth gear and movable internal tooth gear is n=1. As a result, the sun gear and planetary gear always mesh at only one place (one tooth each), the torque applied to the teeth becomes very high and the load on the gears becomes great, leading to problems of reduced durability and difficulties in the miniaturization of the gears due to the requirement for increased tooth strength. Practical implementation is therefore difficult particularly when size is reduced. In addition, when the tooth count difference between the fixed internal tooth gear and movable internal tooth gear is 2 to 4, the number n of planetary gears that can be provided is also 2 to 4, but the gear reduction ratio drops as the above tooth count difference increases. More specifically, using three planetary gears and a tooth count difference of 3 between the fixed internal tooth gear and movable internal tooth gear, the achievable gear reduction ratio of approximately 231 (the value shown in the previously described example) is the design limit. Furthermore, considering variation in tooth profile when the gear module is small, the gear reduction ratio is often only about half that or approximately 100 for safety in an actual, practical implementation. The gear reduction ratio of a gear train using a planetary gear train mechanism is therefore often normally approximately 100, and even in extreme configurations using gear forms with a small module it is still only possible to achieve a practical gear reduction ratio of less than 200.
On the other hand, if the constraint of having the planetary gears z1=z2 is removed, the number of planetary gears n is not necessarily dependent upon the tooth count difference between the fixed internal tooth gear and the movable internal tooth gear. Furthermore, according to equation (1) it would seem that an infinitely high gear reduction ratio could be achieved. However, in order to actually construct a small gear mechanism, three conditions must be met: it must be possible to actually form gear forms with the gear module required by the size; it must be possible to achieve the required operating strength (rigidity) in the teeth of the gear forms; and in the positioning of the gears it must be possible to overcome the reduced backlash resulting from the miniaturization of the gear train when interlocking the gears to each other in the assembling the gear train.
The present invention solves the problems described above. An object of the invention is to achieve a configuration for a planetary gear train mechanism suitable for the miniaturization of a gear module that does not introduce problems to its operation or assembly, even when the gear forms of the gears are used in the construction of a small gear module, and thereby provide a gear train that can be made smaller than the prior art while providing a sufficient gear reduction ratio.
A gear train according to the present invention for resolving the above problems has the following items: a sun gear with a tooth count of e; n planetary gears (where n is a natural number of 2 or more) with each planetary gear having a first gear part with a tooth count of z1 meshing with the sun gear and a second gear part with a tooth count of z2; a fixed internal tooth gear with a tooth count of I1 meshing with the first gear part of the planetary gears; and a movable internal tooth gear with a tooth count of I2 meshing with the second gear part of the planetary gears. The gear train according to the present invention is further characterized by having each of the tooth counts e, z1, z2, I1, and I2 be a multiple of n.
With the present invention it is possible to configure a gear train that has high durability despite miniaturization and that can be reliably and easily assembled because the first gear parts of the n planetary gears between the sun gear and the fixed internal tooth gear can be assembled meshing at equal intervals around the axis, and because the second gear parts of the n planetary gears can be reliably assembled meshed in the same way with the movable internal tooth gear. Thus, a very well balanced condition can be achieved in the planetary gear mechanism even if the gear train is small (i.e. even if the gear module of the gear forms is small) because the phases, i.e. the alignment, of the gear forms at equidistant points dividing the circumference of each gear in n equal parts are mutually matched as a result of the tooth count e of the sun gear, the tooth count z1 of the first gear parts of the planetary gears, the tooth count z2 of the second gear parts of the planetary gears, the tooth count I1 of the fixed internal tooth gear, and the tooth count I2 of, the movable internal tooth gear all being a multiple of n (where n is a natural number of 2 or more). Furthermore, because the present invention has only one constraint requiring that the tooth count of each gear be a multiple of n and because the tooth count of the first gear part and the tooth count of the second gear part of the planetary gears are not required to be the same, the tooth count combinations of a gear train in accord with the present invention can be set more freely and a higher gear reduction ratio can be easily achieved than in the prior art. Moreover, because there are preferably more than two planetary gears, the load on the gears is less as compared with a situation when only one planetary gear is used.
In the present invention n is preferably 2 or 3. Because the section area of the support stud part projecting in the axial direction of the sun gear can be assured in the carrier (or holder) axially supporting the planetary gears as a result of the number of planetary gears being 2 or 3, rigidity loss in the carrier can be suppressed even when the gear train is miniaturized, and the rotational axis of the planetary gear can be reliably supported.
Problems also do not arise in the present invention even when the gear reduction ratio is 200 or greater when the sun gear is the input and the movable internal tooth gear is the output. To achieve a gear reduction ratio is 300 or greater with the above described prior art, the number of planetary gears is normally reduced or it will be difficult to achieve a configuration enabling the gears to be assembled. But with a gear train in accord with the present invention, the gear mechanism can be configured so that it can be reliably assembled without reducing the number of planetary gears even in such cases of gear reduction ratios greater than 300. In particular, with the configuration of the present invention it is possible to achieve a high gear reduction ratio such as noted above and at the same time configure a gear train having sufficiently practical durability and balance even when using gears having a gear form with a small, clock-size module (for example, a module with at least one gear being 0.1 mm or less) to enable device miniaturization. Specifically, it is possible to provide a gear train that can withstand practical use even if the above-noted gear reduction ratio is 400 or greater.
A gear train according to the present invention is easy to achieve even when a module of a combination of at least one gear of a gear form of the sun gear, the planetary gear, the fixed internal tooth gear, and the movable internal tooth gear is 0.1 mm or less. Normally in the prior art when a module is 0.1 mm or less, there are cases in which gear assembly is not possible if the phase, alignment, of meshing between the multiple planetary gears and the sun gear, fixed internal tooth gear, or movable internal tooth gear does not match because there is little tolerance when the gears mesh. By contrast, because meshing of the plural planetary gears of a gear train in accord with the present invention is always in the same phase, i.e. alignment, the gears can be reliably and easily assembled even if the module of at least one gear form is 0.1 mm or less.
In the present invention the fixed internal tooth gear is preferably formed integrally to the inside surface of a housing of the gear train. By forming the fixed internal tooth gear integrally to the inside surface of the housing, the parts count can be reduced and the overall gear train can be further miniaturized.
In the present invention the pitch diameter of the second gear part of the planetary gear is smaller than the pitch diameter of the first gear part. As a result of the pitch diameter of the second gear part being smaller than the pitch diameter of the first gear part according to the present invention, the pitch diameter of the fixed internal tooth gear mating with the second gear part can be reduced, and as a result the gear train can be yet further miniaturized when the fixed internal tooth gear is disposed inside the housing, and even more particularly when the fixed internal tooth gear is formed integrally to the inside surface of the housing.
It should be noted that each gear train according to the present invention is preferably configured as a drive device by integration with an electric motor such as a motor or other drive source. A gear mechanism that can be miniaturized and achieve a high gear reduction ratio as in this invention is extremely advantageous as a small drive device that can be built in to portable (electronic) devices. It is desirable for use as a portable drug dispensing device or a cell phone vibrator, for example.